A woman cools off with fans and mist put out for spectators as a heat wave continues to sizzle at the Australian Open on Wednesday. Photo: AFP

If the mercury reaches its predicted peak on Friday it will be the first time Melbourne has experienced four consecutive days of temperatures above 41 degrees.

Duty forecaster Geoff Feren said the maximum would be reached by late in the afternoon before a dramatic and welcome change with the temperature expected to plunge by 10 degrees within half an hour, possibly in just 10 minutes.

Spectators at the Australian Open have resorted to all sorts of tactics to shelter from the brutal heat. Photo: AFP

Squally winds will sweep in the change about 7pm in Melbourne, Mr Feren said.

Saturday and Sunday are both forecast for a maximum temperature of 23 degrees with Melbourne set for a string of milder days next week in the mid-20s but Victorians in the north-east will continue to suffer with a 42 degree maximum forecast for Albury on Saturday followed by 37 degrees on Sunday.

Lightning strikes as play continues on court 22 at the Australian Open. Photo: Joe Armao

A severe weather warning issued just before 3.15pm on Thursday, advised that thunderstorms were likely to produce heavy rainfall, damaging winds, hailstones and flooding in the late afternoon and early evening with Melbourne, Geelong, Ballarat, Maryborough, Traralgon and Barinsdale expected to be hit by storms.

On Thursday, Victorians braced for blackouts as the mercury climbed to a dangerous 44 degrees, leaving emergency services aghast that some safety warnings continue to be ignored.

Paramedics were called to heat-related emergencies as firefighters battled blazes - many sparked by lightning - across the state ahead of another statewide total fire ban on Friday.

As of midday on Thursday, paramedics had been called to attend to 35 children who had been left in cars in sweltering conditions since Monday.

Ambulance Victoria spokesman Paul Bentley said the lives of 13 children were put at risk yesterday when they were found in vehicles, including two children whose father left them in a car while he visited a pub, on a day when Melbourne's maximum temperature reached 41.5.

"It's incredibly frustrating that people are still ignoring the warnings and leaving their children in hot cars," Mr Bentley said.

"Leaving a child in a car on any day is potentially dangerous but leaving a child in a car this week could be catastrophic.

"Tragically, the last two summers we have seen two children die. While not all of [this week's] cases were deliberate, it's important in this weather that people take extra care when they are getting into or out of their car to ensure their children are not accidentally locked in."

Ambulance Victoria has been dealing with almost double the usual call outs for cardiac arrests with 26 recorded on Wednesday and 109 people treated for heat exhaustation across Victoria.

Victorians have been asked not to dial triple-0 unless in emergencies with Ambulance Victoria receiving 2016 emergency calls on Wednesday, up from an average of about 1500. An Ambulance Victoria spokeswoman said more than 700 calls made to triple zero had turned out not to be emergencies.

Transport woes

Commuters were warned of a difficult journey home, with scores of trains and trams predicted to fail as the temperature soared into the mid 40s.

Metro had urged commuters who could leave work early to do so. Melbourne's rail operator expects to have to take many of its trains out of service this afternoon as the extreme heat takes its toll.

"Due to the ongoing extreme heat, Metro is urging all of our customers to leave the CBD as early as practicable," spokesman Daniel Hoare said.

"We are managing a very difficult situation and we urge all of those who can travel before the afternoon peak to please do so, where possible. We understand the heat is already making life difficult for all Melburnians and we thank you for your ongoing patience."

Power supply

As Victorians rely on airconditioners to stay cool, the Australian Energy Market Operator, which operates the national electricity market, said it is continuing to "closely monitor electricity supply and consumption over".

Earlier this morning, it warned of possible supply shortfalls between 2pm and 4.30pm in Victoria.

Brunswick residents have reported widespread blackouts on Thursday evening, just two days after households in the inner-city suburb were left without power for several hours.

Premier Denis Napthine said Victoria got very close to its power capacity on Wednesday and urged Victorians to switch off unnecessary appliances.

"I think yesterday we had a particularly high risk situation, where we got very close to the maximum use of power according to our generation capacity and we nearly went over the red line, and we nearly had to look at load shedding," he told radio station 3AW.

Dr Napthine said the problem was exacerbated on Wednesday by one of the four generators at Loy Yang A power station breaking down, and the Basslink cable between the mainland and Tasmania not operating at full capacity for technical reasons.

He said he had been advised the Loy Yang A problem should be fixed on Thursday.

Energy and Resources Minister Nicholas Kotsiras said there had been improvements today to the total electricity supply available to Victoria.

He said load shedding - deliberately switching off electricity to parts of the network - was not expected today but it could not be ruled out. He said no load shedding had taken place so far.

"The potential exists for localised power outages related to bush fires, lightning strikes and other impacts on the network system," he said.

"If this occurs, distribution companies would work to restore supply as they would normally."

He said Victorians could help the situation by turning off non-essential appliances and adjusting their airconditioning to 21 or 22 degrees.

But thousands of Victorians were without power on Thursday morning with power companies across the state reporting more than 2350 homes and businesses without power about 10am and warning that number could increase as the mercury climbs in the afternoon.

On Wednesday, the Australian Energy Market Operator informed the Victorian Government that it could be forced to cut supply to parts of the state, potentially leaving up to 100,000 homes and businesses without electricity.

Asked on Thursday about which suburbs would lose power first and how Victorians could prepare for electricity cuts, Energy Minister Nicholas Kotsiras said that would be determined in consultation with electricity suppliers and AEMO.

He said any power failures would not affect hospitals, nursing homes, public transport or emergency services.

Health warning

The Department of Health has issued a heat health alert and a spokesman said it would be looking at the number of deaths recorded during the heatwave to establish if this week's baking temperatures had led to a spike in the state's death rates.

Heat health alerts became standard practice following the heatwave in 2009 that preceded Black Saturday, which was believed to have contributed to the deaths of more than 350 people.

Health scientist Veronica Miller from Curtin University, an expert in hydration and heat stress, advised people to keep drinking water throughout the day.

She said people who failed to drink enough fluids were like cars without enough radiator fluid and would overheat just as easily.

Dr Miller said even when people were not active on hot days they could lose about 300mls of water in an hour and if physically active up to a litre.

She said people who felt fatigued, dizzy or were suffering from a headache could be experiencing the first signs of heat stress and advised them to stop any physical activity, get out of the sun, put their feet in a pool of water or have a cool shower, sit in front of a fan and drink plenty of liquids.